In this specification where a document, act or item of knowledge is referred to or discussed, this reference or discussion is not an admission that the document, act or item of knowledge or any combination thereof was at the priority date, publicly available, known to the public, part of common general knowledge; or known to be relevant to an attempt to solve any problem with which this specification is concerned.
While the process of the present invention is described with specific reference to sites contaminated by mining it will be appreciated that the process is not so limited in its application but can be used for any site having soil contaminated by one or more metals. Furthermore, the process of the present invention is not limited to the metallophyte plant species mentioned herein but can utilise any metallophyte plants that are known or that become apparent in the future, provided they are otherwise suited to the relevant site.
Elevated levels of metals within landscapes, particularly heavy metals such as cadmium, copper, mercury, manganese, lead, nickel, zinc, cobalt, uranium and the metalloid arsenic are often an unavoidable consequence of mining and other industrial activities. Sites contaminated with heavy metals are frequently problematic for the re-establishment of a sustainable vegetation cover and are prone to erosion, leading to a high risk of contaminant dispersion into adjacent areas, despite the many approaches that have been used to address heavy metal contamination and/or excessive costs involved. It is recognised that chronic exposure to these toxic elements is responsible for many serious human conditions such as cancers and degenerative diseases (Lasat, 2000; Tu et al., 2000; Dahmani-Muller et al., 2001; Khan, 2001; Qiao and Luo, 2001; McGrath et al., 2002; Wu et al., 2004).
Phytoremediation
Among the promising methods that developed nations are increasingly using for treating contaminated landscapes is phytoremediation (Ebbs and Kochian, 1997; Huang et al., 1997; Blaylock, 2000) which involves use of plants and microbiota associated with the roots to remove, contain, or render environmental contaminants harmless (Kirkpatrick et al., 2006). It is currently the best, most cost-effective, low-maintenance and publicly most accepted remediation technique to prevent adverse environmental impacts of former mine sites, (i.e. for large scale mine dumps) (Ghosh and Singh, 2005) but its success is limited due to heavy metal toxicity. Phytoremediation offers a natural solution for the recovery of contaminated sites while simultaneously providing soil surface stabilisation and erosion control. Recent studies in Australia have also shown that metal-adapted plant species (metallophytes) used for phytoremediation are able to ameliorate the toxic environment around their roots by rapidly reducing the pool of water-soluble heavy metals in the soil, allowing the concurrent establishment of less tolerant species (i.e. subsequent natural succession of plant communities) (Whiting et al., 2001). In Europe, the use of metal tolerant plant populations (mostly grasses) to stabilize and revegetate waste is well known. In particular, the use of ecotypes of temperate grasses is a proven technology for stabilizing medium toxicity mine tailings, wastes and sites contaminated by industry (Tordoff et al., 2000). There are many thousand metal tolerant, non-accumulating plant species that might be considered for phytostabilisation (Prasad and Freitas, 2003). All these species restrict the transfer of metals from the soil to their shoots, which reduces the entry of metals into the food chain (Baker, 1981; Massoura et al., 2004).
In Australia, less than 5% of the 110,000 heavy metal contaminated sites have been cleaned up with phytoremediation or other bioremediation techniques. The estimate of more than 110,000 polluted sites in Australia is probably doubled if former arsenic sheep dips are included in the tally (Commonwealth Scientific and Industrial Research Organisation Sustainability Network, 2004). Currently, 80-90% of these contaminated sites are dealt with by expensive excavation of soil and storage of the soil elsewhere. Around 10% of sites are sealed under a layer of concrete, and another 5-10% addressed through soil stabilisation. Less than 5% are cleaned up with bioremediation or other methods. A frequent condition of heavy metal contaminated soils is also a lack of effective vegetation cover, making these soils prone to erosion and therefore with a high risk for distribution into adjacent areas. The potential contamination of drinking water resources, the food chain and air poses threats to both ecosystem and human health. This potential contamination is particularly threatening at those sites under the influence of weather extremes and those situated in or connected to areas of high conservation significance such as Kakadu National Park and the Great Barrier Reef in Australia. The clean-up of metal-polluted soils associated with high environmental risk is thus of highest interest economically as well as for protection of human and environmental health.
Surprisingly, there has been limited research on the use of Australian native plants for phytostabilisation purposes. This is despite (i) Australia being the second most plant biodiverse country in the world (25,000 native species out of a total of 250,000 plant species globally) and (ii) the high level of mining activity in Australia.
Clean up of contaminated sites associated with high environmental risk is still of high priority as these sites pose threats to both ecosystems and human health. Accordingly there is a clear need for a viable technique to decontaminate heavy metals from landscapes and assist native plant establishment even under harsh conditions.